Provide migration instructions from egg.mk to wheel.mk

Thanks to adam@ for preparing the instructions, I added a few more
details.

v23/mklinechecker.go

@@ -325,7 +325,36 @@ func (ck MkLineChecker) checkIncludePythonWheel() {
 			mkline.Warnf("Python egg.mk is deprecated, use wheel.mk instead.")
 			mkline.Explain(
 				"https://packaging.python.org/en/latest/discussions/wheel-vs-egg/",
-				"describes the difference between the formats.")
+				"describes the difference between the formats.",
+				"",
+				"To migrate a package from egg.mk to wheel.mk,",
+				"here's a rough guide:",
+				"",
+				"1. If the distfile contains pyproject.toml,",
+				"look for build requirements and add them as TOOL_DEPENDS",
+				"",
+				"2. If there is no pyproject.toml,",
+				"and there is only setup.py, add:",
+				"",
+				"\tTOOL_DEPENDS+=\t${PYPKGPREFIX}"+
+					"-setuptools-[0-9]*:../../devel/py-setuptools",
+				"\tTOOL_DEPENDS+=\t${PYPKGPREFIX}"+
+					"-wheel-[0-9]*:../../devel/py-wheel",
+				"",
+				"Generally, if setuptools is required to build,",
+				"wheel is also needed.",
+				"",
+				"wheel.mk also provides py-test as TEST_DEPENDS",
+				"and a test target (do-test),",
+				"so remove these if they have become redundant.",
+				"",
+				sprintf("Run %q,", bmake("package")),
+				"which will complain about mismatches in the PLIST.",
+				sprintf("Run %q", bmake("print-plist > PLIST")),
+				"to regenerate the PLIST from the actually installed files.",
+				"The typical differences are that the files in EGG_INFODIR",
+				"are replaced with files in WHEEL_INFODIR.",
+				"Also, there may be new files py.typed appearing.")
 		}
 	}
 }